1. Field of the Invention
The present disclosure relates to a solid insulated switchgear, and particularly, to a switch for a solid insulated switchgear (so-called ‘arc extinguishing unit) having an earth switch, and capable of performing a high insulating performance in an earth opening state in ordinary times.
2. Background of the Invention
The present invention relates to a switchgear capable of performing operations to divide electric lines and to break an electric load in an underground electric power distribution system using a high voltage of several tens of kilo bolts, among electric power transmission and distribution systems.
As such switchgear called ‘multi circuit load break switch’, has been mainly used a gas insulated switchgear having therein a plurality of switches in a tank where arc extinguishing gas formed of sulphur hexafluoride (SF6) has been filled.
However, such gas insulated switchgear is being replaced by a solid insulated switchgear using a solid insulated switch, due to a problem that gas pressure should be maintained, a problem that sulphur hexafluoride (SF6) should be used, the SF6 known as gas causing the global warming and restricted in use in the world, and so on.
The configuration and the operation of the solid insulated switchgear have been disclosed in Korean Patent Registration No. 10-0832331 (title of the invention is “Power transmission apparatus for high voltage load break switch”) filed and owned by the present applicant.
In the conventional art of Korean Patent Registration No. 10-0832331, both a main circuit switch and an earth switch are configured as a vacuum interrupter.
However, the conventional art where a vacuum interrupter is used as an earth switch, the distance between a movable contact and a fixed contact is about 10 millimeters on the earth switch which is in an open state in ordinary times, due to characteristics of the vacuum interrupter. Such short insulation distance between the contacts, may cause a ground fault by insulation breakdown due to the occurrence of sparks, etc. in ordinary times where the earth switch is in an open state, in a switchgear of an underground distribution system used in a power circuit having a very high voltage of several tens of kilo bolts.
In order to protect an operator or a manager of the solid insulated switchgear which can contact an enclosure of an arc extinguishing unit (i.e., switch enclosure) from an electric accident such as an electric shock, has been disclosed a technique where a semi-conductive layer is formed at the switch enclosure so as to be earthed.
FIG. 3 shows a configuration that a semi-conductive coating layer (SCC) is coated on the entire part of an enclosure of an arc extinguishing unit 20, i.e., a switch enclosure in accordance with the conventional art.
However, such configuration has the following problems.
If the semi-conductive coating layer (abbreviated as SCC) is coated on the entire part of the switch enclosure, as shown in FIG. 5 or 7, an electric field may be concentrated between a conductive path and an earth terminal inside the arc extinguishing unit under the state that a ground is opened and an electric current flows through the main circuit.
In order to reduce such concentration of an electric field, a space distance and a surface distance between the conductive path and the earth terminal should be increased. This may result in increase of the size of the switch, i.e., the arc extinguishing unit. This may cause a difficulty in miniaturizing the switchgear.